The
Steam Users Forums offers a solution for owners of the boxed retail version
of Half-Life 2 who are vexed that they are required to have the game CD or DVD
in a drive to run the game. Once you register your key through
Steam, you can also download the game
through the broadband delivery service, after which the game can be played
without the disk present.

I have to imagine that it is true, considering the functionality of Steam. Once the key is mapped to your login, ditch the CD installation, and install the game via Steam. Seems to me like it would have to work.